DIY History | Transcribe | Scholarship at Iowa | Eccentricity of the Sextant by Frederic Furbish, 1893 | Eccentricity of the Sextant by Frederic Furbish, 1893, Page 34

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The principles of the sextant.
The two fundamental principles of physics on which the working of the sextant depends are: -
(1) The angle of incidence equals the angle of reflection.
(2) The angle between the first and last direction of a ray of light which has undergone two reflections in the same plane is equal to twice the inclination of the reflecting surfaces to each other.
The proof of the second rule is as follows: - Let C = the index glass and D the horizon glass. ACF is perpendicular to C and DF is perpendicular to D. BCDE is the course of the ray from object B reflected at C and D. By (1) BCA = ACD and CDF = ED

27
The principles of the sextant.
The two fundamental principles of physics on which the working of the sextant depends are: -
(1) The angle of incidence equals the angle of reflection.
(2) The angle between the first and last direction of a ray of light which has undergone two reflections in the same plane is equal to twice the inclination of the reflecting surfaces to each other.
The proof of the second rule is as follows: - Let C = the index glass and D the horizon glass. ACF is perpendicular to C and DF is perpendicular to D. BCDE is the course of the ray from object B reflected at C and D. By (1) BCA = ACD and CDF = ED